Water and Solutions. The importance of water Important component of solutions Water quality, composition, and pH can drastically affect an experiment.

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Presentation on theme: "Water and Solutions. The importance of water Important component of solutions Water quality, composition, and pH can drastically affect an experiment."— Presentation transcript:

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Water and Solutions

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The importance of water Important component of solutions Water quality, composition, and pH can drastically affect an experiment

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Solution terminology Solution: A mixture in which individual molecules or ions are dispersed in a liquid. Solvent: The liquid that makes up the majority of the solution. ▫e.g., water (aqueous solution) Solute: The minority component of the solution. Often a solid before mixing.

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Measurements of concentration In almost all cases, these are amount of solute per volume of SOLUTION.* Weight-per-volume (e.g., mg/L) Percent (parts per hundred) PPT, PPM, PPB Molar solutions * [The exception is molal solutions (moles of solute per liter of solvent) but we will not discuss them further in this class.]

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Weight-per-volume concentration g/L, µg/mL, etc. How to make up a 10 g/L aqueous solution: ▫Weigh out 10 g of the substance. ▫Put it in a vol. flask or grad. cylinder ▫Add water up to the 1 L mark. If you mix 1 L water with 10 g solute, then the total volume would be >1 L, and the concentration would be <10 g/L.

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Normal (N) solutions Definition: The molarity of hydrogen ion equivalents produced by a compound in solution. (Usually applies to acids/bases.) What does this mean? ▫For many chemicals the molarity and normality are the same ▫1M HCl is the same composition as 1N HCl ▫Same for HNO 3, HF, most organic acids (e.g. HCOOH)

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Differences between molar and normal solutions occur when you are working with molecules with >1 exchangeable proton. ▫Sulfate, phosphate, carbonate ▫18M sulfuric acid (H 2 SO 4 ) is 36N  There are 2 H + ions (protons) per molecule  Multiply the molarity of a solution by the number of H + to get normality ▫5M phosphoric acid (H 3 PO 4 ) is 15N

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Making very dilute solutions When making very dilute solutions (micromolar concentrations, etc.) you may need to design a 2- step protocol. You can’t weigh out <10 mg easily. So use more, make a concentrated stock solution, and then dilute from the stock.

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One other complication Sometimes a concentration is specified in terms of one element in the solution: ▫Make up 1 L of 10 ppm Zn solution ▫That would be 10 mg Zn per liter. ▫But elemental Zn is an insoluble metal! Use zinc sulfate (heptahydrate) – ZnSO4 ▫You’ll need to use more than 10 mg of zinc sulfate to get the 10 mg of zinc that you want. Multiply amount of zinc needed by the ratio of formula weight to atomic weight of Zn.

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Special considerations for strong acids and bases SAFETY: Whenever possible, add acid to water, not water to acid. ▫Important primarily for very concentrated acids ▫Prevents splashing & over-heating of acid “Full-Strength” is not necessarily 100%. ▫Ex.: 100% HCl is a gas! The most concentrated liquid form is a 36% aqueous solution. ▫So when diluting to make 10% HCl, you are starting from 36%, not 100%. ▫Acid concentration chartAcid concentration chart

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Practice problems 1. Describe how you would make up ▫250 ml of 200 mM NaCl. ▫250 ml of a 0.05M glucose solution ▫10 ml of M glucose solution starting from a 0.05 M stock solution 2. How many grams of acetic acid would you use to make 10L of a 0.1 M solution of acetic acid?

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3. What volume of 10 M acetic acid is required to produce 1 liter of 0.5 M acetic acid? 4. Describe how to make a 2 N solution of sulfuric acid.

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5. Describe how to prepare 500 ml of a 0.25M solution of sodium hydroxide from a 1.0M stock solution of sodium hydroxide.

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More Practice Problems For hints and answers to these problems go to: 1.Outline how you would prepare 250 ml of a 500 mM solution of potassium chloride. For this problem only, and not for those that follow, give complete details of what glassware you would use, and the mechanics of making the solution. 2.How would you prepare 100 ml of a 5 µM solution of ammonium nitrate (NH 4 NO 3 )? 3.Pete Paladin measures out 30 ml of 1M sucrose solution, and then adds water to make a total volume of one liter. What is the molarity of the final solution? 4.Outline how you would prepare 1 liter of 10% lactic acid, from a bottle you would probably find in the laboratory supply cabinet. What extra step should you take as a safety precaution? 5.Outline how you would prepare one liter of a solution of 5 ppm nickel. The chemical available to you is nickel chloride (NiCl 2 ·6H 2 0). 6.What is the molarity of nickel in the 5 ppm solution prepared above?